With the continuous improvement of people's living standards, finishing varnish is widely applied to various products (such as automobiles, plastics, and glass products) to improve the decorative effect and protect the ground coat. Conventional finishing varnish mainly includes a resin-based film former and a solvent. The film former is formed by the cross-linking and curing of a polyester resin or an acrylic resin with an amino resin. Featuring good weathering resistance, the acrylic resin is preferable. In general, the solvent employs a volatile organic solvent. However, the finishing varnish has the following disadvantages:
1) Most finishing varnish can be only coated on a rough metal surface, but cannot be coated on the surface of plastics (ABS, PA, PB, PP, PC, PVC, etc.), glass, and alloy base having a smooth surface;
2) The hardness and flexibility of the varnish film cannot be both taken into account, the hardness is generally no more than 3H, and the flexibility is generally no more than 1;
3) The scratch resistance of the varnish film is poor;
4) The solvent resistance, boiling resistance, and perspiration resistance are bad;
5) The solvent is volatile and poisonous, which is harmful to environment and people;
6) The finishing varnish is difficult to spray (the varnish cannot be sprayed repeatedly on the wet film);
7) Pinholes and fisheye are inevitably formed on the surface of the varnish film, thereby resulting in poor evenness; and
8) The finishing varnish has low cost performance, so it is very difficult to popularize.
In summary, the existing finishing varnish has disadvantages of narrow field of application, poor scratch resistance, bad environmental performance, and high costs.
The finishing varnish sprayed on the automotive bodies is the final coating, which affects the whole decorative effect of an automobile, so it is required to have good glossiness. In addition, the coating is exposed to the air, acid rain, and air pollutants, and thus it should have good acid resistance. Currently, a typical car painting process employs a solvent-based varnish, which produces a large amount of volatile organic substances thereby polluting the environment. The environmentally friendly water-based finishing varnish on the market has high production cost and unsatisfactory performance, thereby limiting the application thereof.